• Buy a lock or chain of tool hardened steel
• Cheap brittle locks can be broken easily.
• Lock your whole bike including the frame and wheels

There are several kinds of locks and security devices available on the market.  It's important that you use a lock that is made of tool-hardened steel.  This means cutting, sawing or drilling tools will not be able to penetrate as easily as they would with ordinary steel.

U Locks: Some U locks are stronger than others; make sure you buy a strong steel-alloy lock. (If the manufacturer offers a warranty or insurance, register the lock and write down the lock's serial number and when you bought it.)

One drawback to U locks: you can't lock up to thick objects such as streetlights: for these, carry a thick cable lock.

Padlocks & Chains: The thicker, the better.  Chain links and lock clasps should be at least 8 to 10mm thick. Look for locks and chains that are case-hardened. This process makes them harder to cut.

Cables: Some cables are actually harder to cut than chains, because they don't snap and thieves can't pry them open. Use a cable at least 1cm thick with a lock as thick, or thicker.

Lock the Whole Bike: You should put your chain, cable, or U locks through your frame and both wheels-taking the front wheel off if you have to. Never lock through your wheel without locking the frame!